The present invention relates to semiconductor processing and integrated circuits. More particularly, the present invention relates to semiconductor devices and circuits that have a low tolerance to ionizing radiation, and methods for manufacturing the same.
Older semiconductor processing technologies produced integrated circuits that were highly susceptible to damage from ionizing radiation. Such ionizing radiation is emitted from a multitude of galactic sources (e.g., the Sun, etc.) and exists above the ionosphere, and is also emitted when nuclear weapons are detonated. The important consequence of such radiation susceptibility is that these integrated circuits were not well suited for use in satellites or in military applications. They could, therefore, be freely sold and exported without the fear that they would be used militarily against the United States and its allies.
In contrast, state-of-the-art semiconductor processing technologies currently produce integrated circuits that are highly tolerant to damage from ionizing radiation. Such tolerance results, among any other reasons, from a decrease in semiconductor-feature size (e.g., interconnect line width, etc.) in integrated circuits. In particular, the xe2x80x9cgate oxidexe2x80x9d or xe2x80x9cgate insulatorxe2x80x9d in field effect transistors (FETs) has thinned to the point it is inherently tolerant to ionizing radiation.
The relatively high radiation tolerance of state-of-the-art circuits is no benefit to most users and for most applications. This characteristic does, however, allow such circuits to be used in aerospace and military applications. In fact, such circuits may be so radiation tolerant that Department of Defense export restrictions (ITAR) are implicated. Such export restrictions are financially detrimental to a commercial CMOS fabricator since they result in added expense and a shrunken market.
Thus, a need exists for a method that increases the susceptibility of semiconductor devices and integrated circuits to ionizing radiation so that they can be freely exported.
Some embodiments of the present invention provide a method for manufacturing semiconductor devices and circuits that have increased susceptibility to ionizing radiation. The inventive methods use well-known processing techniques. In fact, with the exception of a few processing steps, the inventive methods follow conventional CMOS processing methodology. As such, while devices and circuits that are produced in accordance with the present teachings are more susceptible to damage/aberrant operation from ionizing radiation than devices and circuits fabricated via conventional processes, they nevertheless possess the advantages of contemporary processing technologies (e.g., small feature size, etc.).
In accordance with the illustrated embodiment of the present invention, a radiation susceptible device, such as a FET, is fabricated by forming a screen layer on a substrate; implanting said substrate with a dopant through said screen layer; selectively removing a portion of said screen layer; and forming an electrically-insulating layer on said screen layer. Conventional CMOS processing steps are used to complete the devices. A device exhibiting increased radiation susceptibility is formed at a location at which the screen layer is not removed. A device exhibiting a xe2x80x9cstandardxe2x80x9d or otherwise unaffected susceptibility (relative to conventional CMOS processing procedures) is formed at a location at which the screen layer is removed.
In a further embodiment, a method of operating an integrated circuit in accordance with the present invention comprises: processing signals with a utile device; and interfering with the operation of the utile device with a safeguard device comprising a transistor having a gate insulator that includes a screen layer, wherein the safeguard device interferes with the operation of said utile device when and only when said integrated circuit is exposed to ionizing radiation.
In additional embodiments, the present invention provides an integrated circuit exhibiting increased susceptibility to ionizing radiation, and a method for fabricating such a circuit. An integrated circuit fabricated in accordance with the inventive method comprises (1) xe2x80x9cutilexe2x80x9d devices that provide the functionality for which the circuit was designed, and (2) safeguard devices that impart the integrated circuit""s increased susceptibility to ionizing radiation. In some embodiments, a utile device comprises a first transistor that is disposed on a region of a substrate at which a screen layer is removed, and a safeguard device comprises a second transistor disposed on a region of substrate at which said screen layer is not removed.
In one embodiment of such an integrated circuit, safeguard devices are coupled into the logic of the integrated circuit in such a manner that when the integrated circuit is exposed to ionizing radiation, the safeguard devices irreparably destroy the functionality of the integrated circuit.
In general, the safeguard devices can destroy the functionality of the integrated circuit in two ways. First, one or more safeguard devices can interfere with the logical operation of an integrated circuit by, for example, shorting a signal lead to ground. Second, one or more safeguard devices can interfere with the electrical operation of an integrated circuit by, for example, shorting VDD to ground. This technique works by depriving the utile devices on the integrated circuit of electrical power.